Embodiments of the subject matter disclosed herein generally relate to multi-stage compressors and methods for operating the same. More specifically, the disclosure relates to multistage compressors having a stack rotor configuration.
Multi-stage compressors are widely used for industrial refrigeration, oil and gas processing and in low temperature processes and other uses.
Among the multitude of multi stage compressors of the know type, multi-stage compressors comprising stacked impellers held together by a tie rod are well known. A multistage compressor comprising a stack rotor is disclosed e.g. in US2011/0262284.
FIG. 1 illustrates an axial sectional view of a multi-stage compressor of the current art, and FIG. 2 illustrates an enlargement of a detail of FIG. 1. Said compressor is labeled 100 and comprises an inlet 110A, an outlet 110B, a rotor 111 comprised of a plurality of stacked impellers 112, and a stationary housing 113 housing the rotor 111. The stationary housing comprises a diaphragm 113A wherein each impeller discharges its gas flow to convert the kinetic energy of the gas flow into pressure recovery before returning the gas flow to the next impeller. Each impeller/diaphragm combination is usually referred to as a “stage”. The diaphragm 113A and the rotor 111 are housed in a casing 113B. In the compressor, a gas compression path P (indicated by a dashed line) extending from the compressor inlet 110A to the compressor outlet 110B and through said plurality of impellers 112 and the diaphragm 113A is defined. The compression path P is sealed against the casing, diaphragm and rotor, using suitable seals, e.g. dry gas seals S.
The impellers 112 are held together by a tie rod 114, extending axially through the impellers 112. The first compressor stage comprises a first impeller 112A, while the last compressor stage comprises the last impeller 112B. The rotor 111 comprises also two terminal elements 115A and 115B provided at the two opposite ends of the plurality of impellers 112. The two ends of the tie rod 114 are constrained to the terminal elements 115A-115B.
More in particular, the hubs of the impellers 112 have through holes 116 wherein the tie rod 114 is made to pass. The holes 116 are dimensioned so as to leave a clearance 117 between the tie-rod 114 and the impellers 112.
With particular reference to FIG. 2, each impeller 112 comprises two opposite toothed flanges 118 meshing with respective toothed flanges of two respective adjacent impellers 112 or, in the case the impeller is the first or the last impeller of the impellers stack, respectively with a toothed flange of an adjacent impeller 112 and the toothed flange 119 of one of the terminal elements 115A, 115B.
To avoid gas leakage from the compression path P to the clearance 117, seals 120 on the meshing areas 121 of the teeth are provided.
The gas compressor comprises a balancing line 122 (indicated by a dash-dot line) for balancing the axial thrust of the impellers on the rotor bearings. More in particular, the compressor comprises a balancing drum 123 formed on the terminal element 115B. The balancing drum 123 separates a balancing zone 124 from a zone in fluid communication with the outlet of the last compressor stage. The balancing zone 124 is fluidly connected with the inlet of the first impeller 112A, such that the pressure in the balancing zone 124 is substantially equal to the pressure at the inlet of the first impeller 112A. The balancing drum 123 is arranged in a cylindrical housing formed in the compressor casing. Between the housing and the drum a labyrinth seal 123A is provided, so that a calibrate gas flow leakage F from the last stage towards the balancing zone 124 is allowed. The pressure difference between said balancing zone 124 and the opposite face of the balancing drum facing the last stage impeller 112B generates an axial thrust against the balancing drum. The axial thrust on the balancing drum 123 counterbalances the axial thrust generated on the impellers by the process fluid flowing through the compressor. The balancing line 122 is formed by a pipeline, which is usually external to the casing of the compressor.
The compression process provokes a temperature increase of the processed gas flowing through the compressor. During startup, machine components are usually at ambient temperature and are heated up by the processed gas until a steady temperature condition is achieved. In the compressors having a stack rotor as described with reference to FIGS. 1 and 2, the impellers heat faster than the tie rod. This leads to high temperature gradients between the tie rod 114 and the impellers 112 during the startup transient phase. Due to this high temperature gradient, high thermal stresses are generated, which can shorten the life of the compressor or provoke malfunctioning.